1. Field of the Invention
The present invention relates to interconnections between electrical components. In particular, the present invention relates to interconnections and methods that may be utilized to overcome the negative impact of high inductance indigenous to interconnections (such as bondwires or vias through substrates) between components utilized in microwaves applications.
2. Background of the Invention
An important consideration in microwave design engineering is dealing with unwanted inductance. Inductance becomes increasingly common as the frequency of an alternating current increases. At microwave frequencies, this phenomenon becomes a major consideration in the design of electronic equipment. Any length of wire has some inductance. As with a transmission line, the inductance of a wire increases as the frequency increases. Wire inductance is therefore more significant at microwave frequencies than at lower frequencies. As a result, in microwave applications the frequency of any circuit can be altered by inductance, degrading the performance of the equipment.
Typically, individual microwave components are usually connected together by mounting the components with epoxy or by soldering them onto metal traces on a substrate. For larger systems, metal traces on one substrate must be connected to the metal traces on another substrate. A common way of accomplishing this is with small bondwires, bonded (either with an ultrasonic scrub or with thermo-compression) from a metal trace on one substrate, over a gap, to a metal trace on another substrate. The wirebond is typically 1 mil (0.001 inch) in diameter, and may be anywhere from about 10 mils to 50 or even 100 mils or more in length. While 0.10 seems minimal, it can be an appreciable fraction of a wavelength. For example, at 10 GHz, a wavelength is about an inch which means the bondwire can be about 1/10 of a wavelength long. This can have a serious negative impact on the fidelity of a microwave signal.
Usual methods of dealing with high bondwire inductance include: (1) making the bondwires shorter; (2) arranging a plurality of bondwires in parallel; and/or (3) “matching” the inductance of the bondwire by resonating it with a small capacitance. Limitations to each of these approaches exist. Bondwire length typically must be at least a certain length for mechanical reasons, such as allowing for thermal expansion and contraction of the substrates. Arranging wires in parallel is limited by the mutual coupling that inevitably exists between wires if they are close together, and by other effects if the wires are spread out too much. Resonating or matching the bondwires is limited to achieving a certain amount of bandwidth.
There have been numerous research studies which have pertained to controlling inductance in transmission lines. For example, there is known a “distributed amplifier” which distributes capacitance across a transmission line to produce an amplifier with greater bandwidth, which is described in an abstract by Ginzton et. al., (“Distributed Amplification”, Proc. I.R.E., v. 36, pp. 956–969, 1948). The canonical approach for the distributed amplifier is to use a constant impedance transmission line for the input and output. However, the distributed amplifier is concerned with distributing capacitance, rather than inductance. Furthermore, the distributed amplifier utilizes amplifying elements and transmission line terminations.
Another reference is the microwave circuit configuration known as a “traveling wave power divider/combiner.” It is also sometimes called a “chain” combiner (Russell, Kenneth J., “Microwave Power Combining Techniques”, IEEE Trans. on MTT, vol. MTT-27, pp 472–478, May 1979). This approach varies the impedance of a transmission line as a portion of the energy is sent in a different direction. However, this design is based on an assumption that each energy tap of the traveling wave power divider is expected to have a good impedance match, not a high inductance. Moreover, each tap is separated from the next by a nominal 90 electrical degrees which can be prohibitively larger for many applications. Also, in this approach, isolation resistors are typically used for the traveling wave power divider.
Another technique involves matching the interconnect inductance with shunt capacitance. This technique addresses the same performance issues by simply providing paralleled inductances and matched elements applied to either end. This approach was published by Nelson, Steve, Marilyn Youngblood, Jeanne Pavia, Brad Larson, and Rick Kottman, “Optimum Microstrip Interconnects, 1991 IEEE MTT-S Digest, pp 1071–1074. This method for dealing with unwanted inductance has been shown to be effective, but, at a substantial cost of bandwidth.
Moreover, the performance limitations produced by individual interconnects were examined in some detail by R. M. Fano in his paper “Theoretical limitations on the broadband matching of arbitrary impedances,” published in the Journal of the Franklin Institute, vol. 249, Jan. 1950 pp 57–83 and Feb. pp 139–155. Nevertheless, the aforementioned references still do not teach or suggest a solution towards overcoming microwave application interconnections having high inductances.
It would be advantageous and desirable to provide an interconnect and method of interconnected components which overcome the negative impact of high inductance indigenous to interconnection elements utilized in microwave applications. Moreover, it would be beneficial to provide an interconnect that can be cost-effectively manufactured while delivering optimal performance.